Chemotherapy is associated with differential survival outcomes in apocrine carcinoma: A nationwide cohort study from the Japanese National Clinical Database–Breast Cancer Registry

BACKGROUND: Apocrine carcinoma (AC) is a rare breast cancer subtype, and the benefit of adjuvant chemotherapy for early-stage disease remains uncertain. Prior studies suggest limited chemotherapy responsiveness in triple-negative AC (TNAC), whereas evidence in pure HER2-positive AC is scarce. This study described outcomes in AC using a national registry and examined outcomes by systemic treatment across subtypes. METHODS: Using the Japanese National Clinical Database-Breast Cancer Registry, we identified 2817 patients with AC from 2004 to 2019. Patients were categorized as pure HER2 AC or TNAC. The primary endpoint was distant metastasis-free survival (DMFS); overall survival (OS) was also assessed. Survival outcomes were analyzed using Kaplan-Meier and Cox models. RESULTS: Adjuvant chemotherapy was associated with longer DMFS in the overall cohort (hazard ratio [HR], 0.70; 95% confidence interval [CI], 0.51-0.96). Subtype-specific analyses showed significantly longer DMFS (HR, 0.26; 95%CI, 0.12-0.57) and OS (HR, 0.18; 95%CI, 0.06-0.57) in patients who received chemotherapy in the pure HER2 AC group; 5-year DMFS was 96.1% with chemotherapy versus 86.8% without. In contrast, no significant association between chemotherapy and DMFS (HR, 0.84; 95%CI, 0.60-1.18) or OS (HR, 0.84; 95%CI, 0.55-1.28) was observed in TNAC, with similar 5-year DMFS with and without chemotherapy (91.1% vs. 89.2%). CONCLUSION: In this large population-based cohort of early-stage AC, the association between adjuvant chemotherapy and outcomes differed by subtype. Chemotherapy was associated with better outcomes in pure HER2 AC, whereas no clear association was observed in TNAC. These findings suggest that treatment strategies for AC may need to consider subtype-specific differences.